1. Field of the Invention
This invention relates to electrical circuitry feeding DC bias and AC signal power to a load, and more particularly, to a battery feed circuit useful in a telephone system for supplying direct current from a central office to a telephone subscriber loop.
2. Prior Art
In a telephone system power must be fed from a central office to its various subscribers by subscriber loops. The equipment of the subscriber, such as a telephone, must receive enough electric current to function properly. The electric circuit in the central office supplying the current to the load formed by the transmission line and the subscriber equipment is known as a battery feed circuit.
The battery feed circuit supplies a DC current to the subscriber. Upon the DC current are superimposed AC signals, usually of audio frequency, by which information is conveyed to the subscriber from the central office and from the subscriber to the central office and onward.
To protect the battery feed circuit and other central office equipment, DC source impedances are used in the circuit to avoid undesired high currents which may occur upon a short circuit in the subscriber loop. However, a straightforward solution with the use of a high source impedance will reduce the maximum length of the transmission line forming the subscriber loop. Some of the different approaches used in trying to match the DC source impedance with the conditions imposed by the load of the transmission line are described in U.S. Pat. No. 4,176,255, issued Nov. 27, 1979 by J. A. Rudisill.
A related problem facing battery feed circuits is that of power dissipation. A battery feed circuit manufactured as an integrated circuit, would be physically compact and reliable, but integrated circuits have a low ability of dissipating power. A battery feed circuit with low power would satisfy the requirements imposed by an integrated circuit. Moreover, it is desirable that the overall battery feed system, including the battery feed circuit and the transmission line, dissipate as little power as possible for energy savings.